The simple MEL answer is that normally everything on an airplane has to work every flight. The FAA and the aircraft manufacturer understand this isn't going to happen so they publish a Minimum Equipment List saying what can be inop, for how long and what procedures and limitations have be in place to allow the plane to fly.

Say the APU is inop. The MEL will say it has to fixed in 10 days on a B727. If there is only one nav light in a wing tip and it's not working, the MEL will say how many days the plane can fly and also have a limitation that that the plane can not fly at night. If a fuel tank boost pump is inop there will be a time limit, a note that the other boost pumps must work and a requirement to carry extra fuel and treat it as unusable to prevent the remaining pump/pumps from importing during takeoff and landing.

The MEL for a 747 is about 5 inches thick.